Method and apparatus for imaging the choroid

ABSTRACT

A method and apparatus for imaging the choroid is described. A plurality of choroidal images is captured. The plurality of images is aggregated to generate a single aggregated image with good contrast. Prior to any aggregation, the images may be aligned to remove any displacement caused by saccadic motion between one frame and another, and any images degraded by a blink of the patient can be identified and discarded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/806,335 filed Dec. 21, 2012, which is a national phase entry ofInternational Patent Application No. PCT/CA2011/050389, filed Jun. 23,2011, which application claims the benefit of priority of U.S.Provisional Patent Application No. 61/358,683 filed Jun. 25, 2010. Theseapplications are incorporated herein by reference.

FIELD

The present disclosure relates generally to a method and apparatus forimaging an eye. More particularly, the present disclosure relates to amethod and apparatus for imaging the choroid.

BACKGROUND

The use of fundus imagers and ophthalmoscopes is well established as ameans to non-invasively examine the retina of the human eye to aid inthe detection and identification of ocular pathologies. Such instrumentsinject light through the pupil of the eye and collect light reflectedfrom the retina and passing back through the pupil.

This type of observation is well suited for the examination of the upperlayers of the retina and routinely shows a bright optical nerve head(ONH) and blood vessels leading back to the ONH.

The layer below the retina, the choroid, remains substantially hiddenfrom such observations because most of the incident light is reflectedor absorbed before it reaches the choroidal layer. The choroid, alsoknown as the choroidea or choroid coat, is a vascular layer containingconnective tissue of the eye lying between the retina and the sclera, asshown in FIG. 1. In humans the thickness of the choroid is about 0.5 mm.The choroid provides oxygen and nourishment to the outer layers of theretina.

Non-invasive observation of the choroid can provide useful andsignificant information pertaining to a number of ocular diseases, suchas choroidal melanoma and choroidal neovascularisation.

SUMMARY

A method and apparatus for imaging the choroid is disclosed.

In a first aspect, the present disclosure provides a method for imagingthe choroid. The method comprises transclerally illuminating the choroidby propagating a substantial portion of incident light through thesclera of the eye. The incident light has a wavelength spectrum in thenear-infrared region and the sclera acts as a waveguide to guide,substantially through total internal reflection, the incident light toilluminate the choroid from the rear of the eye. A plurality oftransmission images of the choroid is obtained by collecting the lightpassing out through the choroid and the pupil of the eye using an imagesensor. The plurality of transmission images of the choroid isaggregated into a single aggregated image of the choroid.

In a further aspect, there is provided an apparatus for imaging thechoroid. The apparatus comprises an illumination source, an image sensorand a processor. The illumination source transclerally illuminates thechoroid by propagating a substantial portion of incident light throughthe sclera of the eye. The incident light has a wavelength spectrum inthe near-infrared region and the sclera acts as a waveguide to guide,substantially through total internal reflection, the incident light toilluminate the choroid from the rear of the eye. The image sensorcollects the light passing out through the choroid and the pupil of theeye to obtain a plurality of transmission images of the choroid. Theprocessor is configured to aggregate the plurality of transmissionimages of the choroid into a single aggregated image of the choroid.

Other aspects and features of the present disclosure will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments in conjunction with theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described, by way ofexample only, with reference to the attached Figures, in which:

FIG. 1 is a cross-sectional side view of an eye;

FIG. 2 is a cross-section of the sclera and the cornea showing typicallight paths in accordance with an embodiment;

FIG. 3 is a typical polar diagram of a scattering medium showing apreference for forward scattering over backward scattering;

FIG. 4 is a front view of the eye and surrounding skin belowillustrating regions for injecting light for imaging the choroid inaccordance with an embodiment;

FIG. 5 is a side view of a face showing light launched below the eye (ortowards the edge) and the light passing out through the pupil forimaging the choroid in accordance with an embodiment;

FIG. 6 is an image of the choroid obtained from illumination of thechoroid from light injection through the lower eyelid;

FIG. 7 is an image of the choroid obtained from illumination of thechoroid from light injection through a different portion of the lowereyelid, where the upper left portion appears to be back lit, while thelower right portion appears front lit as in a conventional fundus image;

FIG. 8 is a flow chart illustrating a method of imaging the choroid inaccordance with an embodiment; and

FIG. 9 is a flow chart illustrating a method of obtaining a plurality ofimages of the choroid and converting the plurality of images in to asingle image of choroid to improve signal-to-noise ratio, contrast, andto reduce motion blur in accordance with an embodiment.

DETAILED DESCRIPTION

Generally, the present disclosure provides a method and apparatus forimaging the choroid.

In various example embodiments the following features may be includedcollectively or alone or in any combination thereof.

The present disclosure provides a method for imaging the choroid asshown in FIG. 8. The method comprises indirectly illuminating thechoroid of an eye using incident light having a wavelength spectrum inthe near-infrared region (802) and collecting the light passing outthrough the choroid and the pupil of the eye using an image sensor toobtain an image of the choroid (804).

The indirect illumination of the choroid may be accomplished bytransclerally illuminating the choroid. Transcleral illumination of thechoroid may comprise applying the incident light transdermally throughthe upper or lower eyelid or applying the light directly to the sclera.Indirectly illuminating the choroid may comprise propagating asubstantial portion of the incident light through the sclera of the eye,where the sclera acts as a waveguide to guide the incident light to thechoroid. Indirectly illuminating the choroid may comprise illuminatingthe choroid by applying incident light at a plurality of locationssurrounding the eye or surrounding the cornea using one or more lightsources associated with different angles of incidence. The proportion ofthe incident light that propagates through the sclera to the rear of theeye to the incident light that is transmitted through the sclera nearthe front of the eye may be controlled.

The image of the choroid may be obtained by capturing a single image ofthe choroid or by capturing a continuous stream of images of thechoroid. The captured image or stream may include an image of the Halleror Sattler layers of larger choroidal vessels.

An additional light source having a wavelength spectrum in the visibleregion may be provided for identifying the illumination region of theincident light having the wavelength spectrum in the near-infraredregion.

At least one of size, shape, orientation and convergence angle of theincident light may be adjusted to minimize interference from unwantedscattering.

Blood oxygenation level may be measured using at least two imagesobtained at different wavelengths on either side of the oxygen isobesticwavelength in the near-infrared region.

The present disclosure also provides an apparatus for imaging thechoroid. The apparatus comprises an illumination source and an imagesensor. The illumination source indirectly illuminates the choroid of aneye using incident light having a wavelength spectrum in thenear-infrared region. The image sensor collects the light passing outthrough the choroid and the pupil of the eye to obtain an image of thechoroid.

The illumination source may include means to propagate a substantialportion of the incident light through the sclera of the eye for theindirect illumination of the choroid. In addition, the illuminationsource may include means to illuminate the choroid through a pluralityof locations surrounding the eye or the cornea. The illumination sourcemay also include one or more light emitting diodes (LEDs) emitting lightin one or more wavelengths in the near-infrared region. The illuminationsource may be a single discrete source, a distributed source or anaggregation of discrete sources.

The apparatus may further comprise means to capture a single image ofthe choroid or means to capture a continuous stream of images of thechoroid.

The apparatus may further comprise an optical fiber to deliver theincident light having a wavelength spectrum in the near-infrared region.

The apparatus may further comprise an additional light source having awavelength spectrum in the visible region for identifying theillumination region of the incident light having the wavelength spectrumin the near-infrared region.

In the following description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments. However, it will be apparent to one skilled in the artthat these specific details are not required. In addition, it isunderstood that the features described with respect to any exampleembodiment can be included in other embodiments as well, wheretechnically feasible as understood by a person skilled in the art.

As discussed earlier, the choroid is a layer in the vertebrate eye,which lies immediately outside the retina, between the retina and thesclera. The choroid contains blood vessels, which carry nourishment tothe outer layers of the retina, and pigment. The choroid connects withthe ciliary body toward the front of the eye and is attached to edges ofthe optic nerve at the back of the eye. The choroid consists of fourlayers: an outermost layer of large blood vessels (Haller's layer), alayer of intermediate size blood vessels (Sattler's layer), a layer ofcapillaries (choriocapillary layer), and an innermost layer (Bruch'smembrane). The ability to non-invasively image the choroid can provide awealth of information pertaining to ocular health and can act as adiagnostic tool to detect and monitor various ocular diseases.

The method for imaging the choroid comprises indirectly illuminating thechoroid of an eye using incident light having a wavelength spectrum inthe near-infrared (NIR) region and collecting the light passing outthrough the pupil of the eye using an image sensor to obtain an image ofthe choroid. Indirect illumination of the choroid can includeilluminating the choroid transclerally (i.e., via the sclera) either bydirectly illuminating the sclera or by illuminating the scleratransdermally (for example, through the skin surrounding the eye).

Several techniques have been attempted to illuminate the interior of theeye avoiding the use of the pupil as the route of illumination. Suchtechniques involve illuminating the interior of the eye through thewhite sclera layer that surrounds the eye. Prior attempts atilluminating the interior of the eye through the sclera involve thedirect illumination of the sclera from the illumination source, usuallya laser or an incandescent source, for example a tungsten filament. Asthe only part of the sclera that is readily accessible directly is thatin the close neighborhood of the pupil, this limits the potential of theprior techniques. Moreover, the choice of white light as an illuminationsource resulted in substantial patient discomfort and substantialinefficiency as the retinal and choroidal tissues absorb the shorterwavelengths characteristic of white light.

For example, Cohen et al in “Choroidography and Photography of the LongCiliary Nerve and Artery” Arch Ophthalmol—Vol 95 March 1977, p 436,described an imaging technique using visible white light from a tungstenhalogen source and capturing the images in a photographic film, such asKodak TRI-X™, ASA 2000. The light from the lamp was introduced to thepatient via a fiber optic cable, the far end of which was pressedagainst the patient. In this arrangement, the light passed to thechoroid through a transcutaneous (transdermal) and transcleral pathway.

Other prior art systems illuminate the interior of the eye through thesclera without contacting the patient by focusing light on the scleraclose to the eye. Such systems also describe the use of visible lightfor providing uniformity in the illumination of the retina. Theillumination source is typically a lamp such as a xenon, halogen, ormetal-halide lamp, or any other filament, arc or gas lamps and usesspecific optical components to filter out ultraviolet and infraredcomponents of the light used for illumination. As described earlier, useof white light as an illumination source not only results in substantialpatient discomfort, but also may obscure otherwise pertinent informationabout ocular health.

Yet other prior art systems describe the use of fluorescent dyesinjected into a patient's eye for imaging the choroid. The dyes resultin the blood vessels and the optic nerve appearing as bright areas inthe resultant images. However, injecting fluorescent dyes in patientsraises the concern of an anaphylactic reaction in addition to usualsubstantial patient discomfort from the illuminating flash. Moreover thewindow of opportunity is very short being limited by the fluorescencelifetime.

However, in accordance with example embodiments described herein, thechoroid of the eye is indirectly illuminated using incident light havinga wavelength spectrum in the NIR region. The light passing out throughthe pupil of the eye is collected using an image sensor to obtain animage of the choroid. The illumination, for example, may be made to passthrough layers of skin forming the lower eyelid. In other exampleembodiments, illumination may be provided through the skin at eitherside of the eye and/or through the upper eyelid. In other exampleembodiments, illumination may be provided directly to the scleralsurface. The use of fluorescent dyes is eliminated while patient comfortis sustained by the use of light in the NIR region, which issubstantially invisible to the human eye.

In order to prevent excessive absorption of the illuminating light priorto entering the interior of the eye and to enable patient comfort, lightfrom a spectral band in the NIR region of the spectrum is used. Forexample, the spectral band may lie in the region from 750 nm up to 1000nm or longer. The transmission of infrared light through the skin,sclera and eye tissues improves as the wavelength increases. The upperlimit on of the wavelength spectrum is influenced by the spectralsensing properties of the image sensor that is typically a silicon-basedCharge Coupled Device (CCD) or a Complementary Metal Oxide Silicon(CMOS) device, both of which have very low sensitivity above 1000 nm.However, the emerging technology of image sensors that operate at longerwavelengths such as those based on InGaAs technology would enableimaging at longer wavelengths.

In preliminary trials, light emitting diode (LED) sources operating at anominal center wavelength of 890 nm have yielded good quality images ofthe choroid. Good quality images have been similarly achieved at 940 nm.In both cases, the LED used provides light that is partially collimatedto present a full cone angle of divergence of about 40 degrees. The LEDsused have an industry standard 5 mm lensed package.

As light at the NIR region of the spectrum is substantially invisible,some light in the visible region of the spectrum may be provided toallow the operator to view the point of illumination during alignment.The visible light may be turned off prior to image capture in order toprevent unwanted interference.

In some embodiments, the illuminating light can be delivered to thesclera indirectly with or without contact of the illumination source tothe skin or sclera, and with or without a flexible light guide thatdelivers the incident light. Once the incident light is redistributedwithin the sclera and re-radiated from the rear of the choroid, theredirected light passing out through the pupil is collected and is thenrelayed to a focus at the image sensor where the choroid layer is infocus.

The light entering the sclera is partially scattered and then partiallyguided through the sclera towards the back of the eye and the sclera isacting as a lossy waveguide layer redistributing the light to an areadifferent from where it is captured, as shown in FIG. 2. Such awaveguide property is consistent with the structure of the sclera whichis made up irregular collagen fibers that are continuous with the corneaand reach around to the back of the retina, as shown in FIG. 1. Therefractive index of the collagen fibers of the sclera is about 1.45. Therefractive index of the surrounding tissues is near 1.33. Lightpropagating within the sclera in a direction far away from the normal tothe sclera will be totally internally reflected within the scleraprovided the angle of propagation with respect to the local scleralplane is less than the limiting angle. In a step index structure, theangle is given by cos⁻¹ (n2/n1) where n1, n2 are the respectiverefractive indices. However, the sclera has more of a graded structure.Some portion of the light will also be internally scattered as itpropagates within the sclera, and some of the scattered light will be atangles greater than the limiting angle—this portion of the light willpass through the sclera and back-illuminate the choroid. The scleraabsorbs less and scatters less with the longer wavelengths used in theIR part of the spectrum than it would in the visible region of thespectrum. In order to channel the incident light into the sclera suchthat the sclera acts as a waveguide, the location and angle of thelaunching of incident or illuminating light become important asdiscussed below.

The optical properties of the sclera change significantly andmonotonically as the wavelength increases from the visible region intothe NIR region of the spectrum. In particular, the absorption reducesand the scattering reduces, the respective coefficients typicallyfalling by over 50%. Moreover, the relative proportion of forwardscattering to backscattering increases. Thus, using illuminating lightin the NIR allows more light to pass into and through the sclera,especially where the light path is substantially close to the localplane of the sclera where the sclera is acting as a sheet waveguide or alightguide.

FIG. 2 shows a cross-section of the sclera and the cornea showingtypical light paths when illuminated in accordance with embodimentsdescribed herein. FIG. 2 additionally shows the phenomena of refraction,total internal reflection, and scattering as well as the light path outthrough the cornea to the image sensor.

As described earlier, the sclera is a connective tissue made mostly ofwhite collagen fibers. It underlies the choroid posteriorly andcontinues anteriorly where it becomes transparent over the iris andpupil and is referred to as the cornea, as shown in FIG. 1. Lightincident upon the exterior surface of the cornea travels through thecornea and exits the other surface and no light gets trapped within thecornea.

In contrast, light incident, directly or indirectly upon the exteriorsurface of the sclera is scattered in all directions. A portion of theincident light proceeds through the inner surface of the sclera and aportion of the incident light is scattered at an angle consistent withthe waveguide properties of the sclera and is, therefore, trapped withinthe sclera. Another portion of the incident light is backscattered andlost.

The trapped light will however itself be continuously scattered. As aresult, a portion of the incident light passes through the inner surfacewhile the remainder remains trapped and will be partially scattered outfurther along. The overall effect is that the entire sclera acts as adistributed secondary source of light or illumination, including part ofthe sclera at the back of the eye remote from the initial point of entryof the incident light.

Scattering in the NIR is generally not isotropic, but has acharacteristic polar pattern with preferred axes in the forward andrearward directions, especially in the forward direction as shown inFIG. 3.

In order to optimally launch light into the scleral waveguide, theincident light should not impact the sclera at right angles but insteadimpact the sclera at angles close to the plane of the sclera at thepoint of illumination. The optimal launch angle for the illuminatingincident light can be achieved by choosing a suitable location to launchthe light and by directing the light in a preferred limited range ofangles extending over typically plus and minus 10 degrees around thecentral axis of propagation. The size and shape of the illuminatedregion on the skin, and the associated convergence angle and orientationof the incident light are determining parameters of the choroidalillumination. These geometric parameters can be optimized to minimizeinterference from unwanted scattering thereby improving the quality ofthe resulting image. The required beam parameters may be set by, forexample, suitable combinations of lenses and apertures. The use of fiberoptic cable as a convenient transport element for the illuminating lightallows for flexibility of deployment, and can serve to more evenlydistribute the modes within a set mode volume.

The mode volume of a light beam, sometimes called the opticalthroughput, or etendue, is the product of near-field focused area andfar-field solid angle. When divided by the square of the wavelength, theresult is the number of fundamental geometric modes. In indirectlyilluminating the choroid through the sclera, an objective is to maximizethe proportion of incident light that is converted by internalscattering into modes that are guided within the sclera around to therear of the eye. If the incident light is near normal to the scleralsurface, most of the energy will be forward scattered right through thesclera or backscattered from the sclera. Therefore, it is desirable toilluminate employing an incident angle that is far from normal. It isalso desirable to use a mode volume that is highly collimated, that iswhere the spread of angles is small, subject only to the need to allowfor some tolerancing in the alignment. The shape of the illuminated areamay be circular or square or in the form of a partial annulus, or in anyintermediate shape. The size of the illuminated area can be limited soto allow the incident angles at the scleral surface to be substantiallysimilar; typically, this implies an illumination shape limited in thevertical direction to approximately 1 or 2 millimeters. The partialannulus however could usefully extend approximately horizontally byseveral more millimeters. The size of the human eyeball is remarkablyconsistent and the precise location for illumination can use as areference point the pupil of the eye when the eye is fixated upon atarget.

In an example embodiment, the optical fiber has a core diameter ofapproximately 5 mm and is connected firstly to a collection lens andsecondly to a focusing lens. As described earlier, the fiber allows foreasy flexibility during deployment and also tends to mix the rays andfor facilitating an even level of illumination. However, othercombinations of optical elements can be used to accurately define theilluminating beam in terms of shape, size, incident angle andconvergence.

FIG. 4 is a front view of the eye and surrounding skin belowillustrating regions for injecting light for imaging the choroid inaccordance with an embodiment. FIG. 5 is a side view of a face showinglight launched below the eye (or towards the edge) and the light passingout through the pupil for imaging the choroid in accordance with anembodiment.

An advantage of the indirect illumination of the choroid is that thereis no reflection of illumination from the cornea or the viewing lens,nor is there backscattering from other regions of eye tissue beyond thecornea. This avoids the need to block or remove the otherwise dominantcorneal reflection and facilitates the capture of good quality images.

The nature of the images so obtained depends on the illuminationgeometry. In one form of illumination, light injected or launchedthrough the lower eyelid enters the sclera and appears to provideillumination from the back of the eye beyond the retina. As in theexample shown in FIG. 6, images obtained with this type of illuminationhave the choroidal vessels appearing as dark areas, in effect shadowingthe rear sourced illumination, while the other choroidal areas appearbright as they do not greatly attenuate the rear sourced illumination.Also, the dark appearance of the ONH may indicate that there is no partof the distributed scleral light illumination behind the ONH as the ONHphysically disrupts the sclera. Moreover, the ONH is often apparentlysurrounded with a bright annulus, the overall appearance resembling alunar eclipse of the sun.

While this form of illumination also shows in shadow form the arteriolesand venules in front of the retina leading to the ONH, the predominantshadowed vessels are those of the choroid that do not originate at theONH. In example embodiments, the images may be inverted in brightness,analogous to viewing a negative image, prior to presenting tophysicians, who are more familiar with seeing bright vessels fordiagnostic purposes.

In other illumination arrangements, specifically where the light isarranged to initially illuminate a different part of the lower eyelid,the image shows a combination of both the shadow features describedabove and reflection features commonly associated with conventionalfront illumination, as shown in FIG. 7. This suggests that in the lattercase, some of the light passes through the sclera near the front of theeye to provide front illumination, while some light travels around tothe rear of the eye to provide rear illumination. The illuminationthrough different parts or portions is associated with different anglesof the light paths as they intersect with the sclera.

The light collected through the pupil at the image sensor may containportions of light scattered from various paths of lesser interest inaddition to the portion of particular interest scattered from thechoroid. Consequently, the contrast ratio of the raw images may be poor.However, as the image is captured electronically and stored digitally,the images can be processed to enhance the contrast and emphasize thedesired clarity of the image for diagnostic purposes. The ability todigitally enhance the contrast of the images obtained presents anadvantage over prior art techniques that rely on photographic filmsand/or plates.

From the preliminary trials, it may be inferred that the angle ofillumination and the region near the eye where the illumination occurscontrols the proportion of light that propagates through the sclera tothe rear of the eye to the light that is transmitted through the frontof the eye before being reflected back passing out through the pupil tothe collection optics. In some exemplary embodiments, the method forimaging the choroidal vessels may include adjusting the proportion ofincident light that propagates through the sclera to the rear of the eyeto incident light that is transmitted through the front of the eye.

The images may be captured individually or in a continuous stream in avideo format. For clinical purposes, it is more likely that a singlesnapshot image will be satisfactory, where the exposure duration isshort to avoid eye blur through involuntary movements. This iscompatible with the use of a LED illumination source that is easilydriven to provide a momentary flash of illumination. However, prior to asingle image capture, a video image stream may be used to guide anoperator to properly set the illuminating geometry.

In other example embodiments, multiple illumination sources may be usedfor illuminating the eye. For example, rather than use a single discretesource, a distributed source or an aggregation of discrete sources canbe used. A single physical setting to the eye could be used and asatisfactory image may be obtained—not by moving the patient or theapparatus around—but either by taking advantage of a distributed sourceor by electronically activating sources at different positions andangles until certain pre-conditions for obtaining a satisfactory imageis met. The distributed source could of course simply be an aggregationof discrete sources. For example, a typical discrete source is alight-emitting diode (LED) which has a typical emitting area measuring1×1 mm although it may be much smaller. Alternatively, an aggregation ofLEDs may be used to increase the total energy launched in a flash orcreate particular illumination shapes, incident angles and sizes.

The embodiments described herein have at least two significantadvantages over conventional imaging techniques. The use of light in theNIR region of the spectrum, typically from a LED source, allows forefficient access to the choroidal layer as the intermediate absorptionis much less, permitting a better quality of the resulting image. Theuse of electronic image sensors lends itself to advanced digital lightprocessing techniques that further enhances the image through contrastenhancement, for example. In addition, there is no requirement for theillumination source path to be directly in contact with the patient asit is sufficient to allow the illuminating light to pass through the airbefore impinging on or near the eye of the patient. This arrangementremoves concerns regarding hygiene and allergies to materials.

The choroidal images can be used for an initial observation and forcomparing images taken at different time intervals to identify changesin the choroid over time. As mentioned earlier, images of the choroidmay aid in the detection of ocular diseases such as choroidal melanomaand choroidal neovascularisation. In addition, the ability to monitornew vessels developing in the choroid would be significant for trackingocular health. Current techniques do not readily detect certain cases ofoccult neovascularisation and can easily be missed. Early detection ofany adverse development in the choroid can lead to immediate treatmentwith anti-VEGF drugs and lessen the damage to the retina from physicaldisruption and leaking vessels.

In an embodiment, the choroidal imaging technique described herein isalso used to measure blood oxygenation levels. In this embodiment,choroidal images are captured using at least two illuminatingwavelengths on either side of the oxygen isobestic wavelength near 815nm. The images may be captured with two or more image sensorssimultaneously or in time sequence. The relative brightness at anylocation in the images provides an indication of the oxygenation level.

The choroidal images obtained at different time intervals can beregistered to identify disease progression, for example. A method forperforming registration of multispectral images using cross-over pointsand bifurcation points of blood vessel in an eye is described incommonly owned PCT Application CA2011/050038 and U.S. Pat. No. 8,855,386entitled “Registration Method for Multispectral Retinal Images,” thecontents of which are incorporated by reference in their entiretyherein.

A method for quantifying disease progression through retinal healthassessment and management is described in commonly owned PCT ApplicationCA 2010/000785 and U.S. Pat. No. 8,303,115 entitled “Method and Systemfor Retinal Health Management,” the contents of which are incorporatedby reference in their entirety herein.

With improvements in image sensors, the method and apparatus for imagingthe choroid described herein may be further improved. For example,multi-frame alignment techniques may be employed to convert the multipleimages of the choroid to a single choroidal image with improvedsignal-to-noise (SNR), contrast, and reduced motion blur.

Accordingly, the present disclosure provides a method for imaging thechoroid as shown in FIG. 9. The method comprises transclerallyilluminating the choroid by propagating a substantial portion ofincident light through the sclera of the eye. The incident light has awavelength spectrum in the near-infrared region and the sclera acts as awaveguide to guide, substantially through total internal reflection, theincident light to illuminate the choroid from the rear of the eye (902).A plurality of transmission images of the choroid is obtained bycollecting the light passing out through the choroid and the pupil ofthe eye using an image sensor (904). The plurality of transmissionimages of the choroid is aggregated into a single aggregated image ofthe choroid (906).

Typically, capturing images of the choroid requires a long exposuretime, for example, as long as 5 seconds, in order to collect sufficientenergy to create an image with sufficient contrast to render goodquality. During such a long exposure, the eye of the patient willtypically see involuntary movements or saccades. The impact of suchmovements is to blur the image. The frequency of occurrence of suchmovements is such that there is a probability of their occurrence evenat exposures as short as 50 ms (milliseconds), although the probabilityof such occurrences decreases as the exposure decreases.

Recent years have seen the emergence of image sensors employingComplementary Metal Oxide Silicon (CMOS) technology that in severalapplication areas have successfully challenged the former dominance ofimage sensors employing Charge Couple Device (CCD) technology. Anadvantage of the CMOS image sensors is their ability to combine a rapidreadout rate with low readout noise, the combination enabling theproduction of high quality image frames at a high frequency, for example100 Frames Per Second (FPS).

The present disclosure provides an apparatus for imaging the choroid.The apparatus comprises an illumination source, an image sensor and aprocessor. The illumination source transclerally illuminates the choroidby propagating a substantial portion of incident light through thesclera of the eye. The incident light has a wavelength spectrum in thenear-infrared region and the sclera acts as a waveguide to guide,substantially through total internal reflection, the incident light toilluminate the choroid from the rear of the eye. The image sensorcollects the light passing out through the choroid and the pupil of theeye to obtain a plurality of transmission images of the choroid. Theprocessor is configured to aggregate the plurality of transmissionimages of the choroid into a single aggregated image of the choroid.

In an embodiment, a plurality of choroidal images is captured using acamera with CMOS image sensors. For example, 500 images or frames may becaptured, each having an exposure time of, for example, 10 ms. Each ofthese 500 images will have typically no movement blur, but they willeach have a poor contrast because of the relatively small amount ofenergy captured in the short exposure time. The plurality of images maythen be aggregated to generate a single image with good contrast. Priorto any aggregation, the images may be aligned to remove any displacementcaused by saccadic motion between one frame and another.

Alignment of the images may be carried out by first identifying one ormore key feature markers within the images. These markers may beidentified by applying contrast enhancement and spatial filteringtechniques to each image. After each image has been aligned to a commonframe of reference, they may be electronically aggregated to form aresulting high quality composite image that combines good contrast withreduced or substantially no blur.

As previously described, a method for performing registration ofmultispectral images using cross-over points and bifurcation points ofblood vessel in an eye is described in commonly owned PCT ApplicationCA2011/050038 and U.S. Pat. No. 8,855,386 entitled “Registration Methodfor Multispectral Retinal Images,” the contents of which areincorporated by reference in their entirety herein.

Some of the original images will be blurred if they are captured duringa period within which a saccade occurs. In an example embodiment, theseimages may be recognized and discarded. However, as such events are soinfrequent, that their net contribution to the composite image would berelatively small. Similarly, in other example embodiments, imagescaptured when the patient blinks may be recognized and discarded. Ifpoor images are discarded, the total number of good images required toachieve an acceptable composite image quality will be reduced.

The technique described above is applicable not only to images of thechoroid but also to images of the retina that may require a long overallexposure period.

A method and apparatus for imaging the choroid has been describedherein. An advantage of the method and apparatus according toembodiments described herein is that any part of the choroid that can beobserved via the pupil can be imaged. The illumination optics and thecollection optics are different and the illumination and collection arenot performed through the same aperture. Consequently, a wide areaaround the posterior pole can be viewed or imaged with evenillumination. Another advantage of the method and apparatus describedherein over conventional methods is the ability to image transmissivelythrough the choroid. Current optical coherence tomography (OCT) basedmethods only view the nearer layers, while the method and apparatusdescribed herein aids in the visualization of the deeper Haller andSattler layers of larger choroidal vessels. By combining a plurality ofchoroidal images into a single image, improvements in SNR, contrast, andthe substantial elimination of motion blur may also be achieved.

The above-described embodiments are intended to be examples only.Alterations, modifications and variations can be effected to theparticular embodiments by those of skill in the art without departingfrom the scope, which is defined solely by the claims appended hereto.

What is claimed is:
 1. A method to image the choroid of an eye, themethod comprising: transclerally illuminating the choroid by propagatinga substantial portion of incident light through the sclera of the eye,the incident light having a wavelength spectrum in the near-infraredregion and the sclera acting as a waveguide to guide, substantiallythrough total internal reflection, the incident light to illuminate thechoroid from the rear of the eye; obtaining a plurality of transmissionimages of the choroid by collecting the light passing out through thechoroid and the pupil of the eye using an image sensor; aggregating theplurality of transmission images of the choroid into a single aggregatedimage of the choroid.
 2. The method of claim 1, wherein the singleaggregated image of the choroid has improved contrast and reduced motionblur.
 3. The method of claim 1, further comprising: aligning theplurality of transmission images of the choroid based on one or morefeature markers within the images prior to being aggregated into thesingle aggregated image of the choroid.
 4. The method of claim 3,wherein the one or more feature markers are identified by applyingcontrast enhancement and/or spatial filtering to each of the pluralityof transmission images of the choroid.
 5. The method of claim 3, whereinaligning the plurality of transmission images of the choroid includesaligning each image to a common frame of reference.
 6. The method ofclaim 1, wherein one or more images of the plurality of transmissionimages are excluded based on saccadic motion or a patient blinkingduring imaging prior to being aggregated into the single aggregatedimage of the choroid.
 7. The method of claim 1, wherein transclerallyilluminating the choroid comprises applying the incident lighttransdermally through the upper or lower eyelid.
 8. The method of claim1, wherein transclerally illuminating the choroid comprises illuminatingthe choroid by applying the incident light at a plurality of locationssurrounding the eye using one or more light sources associated withdifferent angles of incidence.
 9. The method of claim 1, furthercomprising: adjusting the incident angle of the incident light tocontrol the proportion of the incident light that propagates through thesclera to the rear of the eye to the incident light that is transmittedthrough the sclera near the front of the eye.
 10. The method of claim 1,further comprising: providing an additional light source having awavelength spectrum in the visible region for identifying theillumination region of the incident light having the wavelength spectrumin the near-infrared region.
 11. The method of claim 1, furthercomprising: adjusting at least one of size, shape, orientation andconvergence angle of the incident light to minimize interference fromunwanted scattering.
 12. The method of claim 1, further comprising:measuring blood oxygenation level using at least two aggregated imagesobtained at different wavelengths on either side of the oxygen isobesticwavelength in the near-infrared region.
 13. An apparatus for imaging thechoroid of an eye, the apparatus comprising: an illumination source fortransclerally illuminating the choroid by propagating a substantialportion of incident light through the sclera of the eye, the incidentlight having a wavelength spectrum in the near-infrared region and thesclera acting as a waveguide to guide, substantially through totalinternal reflection, the incident light to illuminate the choroid fromthe rear of the eye; and an image sensor for collecting the lightpassing out through the choroid and the pupil of the eye to obtain aplurality of transmission images of the choroid; and a processorconfigured to aggregate the plurality of transmission images of thechoroid into a single aggregated image of the choroid.
 14. The apparatusof claim 13, wherein the processor is further configured to: align theplurality of transmission images of the choroid based on one or morefeature markers within the images prior to aggregating the plurality oftransmission images into the single aggregated image of the choroid. 15.The apparatus of claim 14, wherein processor is further configured to:apply contrast enhancement and/or spatial filtering to each of theplurality of transmission images of the choroid to identify the one ormore feature markers.
 16. The apparatus of claim 14, wherein processoris further configured to: align each image in the plurality oftransmission images of the choroid to a common frame of reference. 17.The apparatus of claim 13, wherein processor is further configured to:exclude one or more images of the plurality of transmission images ofthe choroid based on saccadic motion or a patient blinking duringimaging prior to being aggregated into the single aggregated image ofthe choroid.
 18. The apparatus of claim 13, wherein the illuminationsource includes means to illuminate the choroid through a plurality oflocations surrounding the eye.
 19. The apparatus of claim 13, whereinthe illumination source includes one or more light emitting diodes(LEDs) emitting light in one or more wavelengths in the near-infraredregion.
 20. The apparatus of claim 13, further comprising: an additionallight source having a wavelength spectrum in the visible region foridentifying the illumination region of the incident light having thewavelength spectrum in the near-infrared region.